There is known a displacement sensor utilizing a position sensitive device (hereinafter referred to briefly as PSD). Such a sensor is shown at 51 in FIG. 10 of the drawings accompanying this specification. This displacement sensor generally comprises a sensor head 52 housing a light-emitting element 53, which may be a light-emitting diode (LED) or a semiconductor laser, and a projection lens 54 on the light emission side and a condenser 55 and a PSD 56 on the light reception side. The operating principle of this sensor is that a detection light beam 58 from said light-emitting element 53 is obliquely incident on the object 57 and the light 59 reflected therefrom is condensed by said condenser 55 and received by the PSD 56. It is so arranged that the axis of the detection light beam 58 is at right angles (.theta.=90.degree.) with the light axis of the condenser 55. Since, in this sensor 51, the amount of displacement of the object 57 is proportional to the amount of shift in the point of incidence on the light-receiving surface of the PSD 56, the signal output of the PSD is linearly dependent on the amount of displacement of the object 57.
In this sensor 51, however, since both the light-emitting element 53 and the PSD 56 are obliquely disposed with respect to the surface of the object 57, the distance s between the light-emitting element 53 and the PSD 56 is greater than the detection distance L so that the bulk of the sensor head 52 is increased of necessity.
Illustrated in FIG. 11 is another known displacement sensor 61, wherein a light-emitting element 64 and a projecting lens 65 are disposed so as to project a detection light 63 perpendicularly towards the object 62 and, with the light-receiving surface of a PSD 66 being disposed perpendicularly with respect to the axis of a detection light beam 63, a condenser 69 for focusing a reflected light 68 on said PSD 66 is disposed in parallel with the axis of the detection light beam 63.
In this displacement sensor 61, the axis of the detection light beam 63 is at right angles with the light-receiving surface 67 of the PSD 66, the amount of shift in the point of light incidence on the light-receiving surface 67 of PSD 66 is not proportional to the amount of displacement of the object 62 so that the signal output of the PSD is not linearly dependent on the amount of displacement of the object 62, thus requiring a correcting circuit. Moreover, the focal plane, indicated at 70, which is formed by the reflected rays 68 on focusing by the condenser 69, does not coincide with the light-receiving surface 67 of PSD 66 as seen in FIG. 11, so that the reflected light 68 from the surface of said object 62 cannot always be focused on the light-receiving surface 67 of PSD 66.
FIG. 12 is a schematic view showing still another known displacement sensor 71. In this displacement sensor 71, a light-emitting element 74 and a projecting lens 75 are disposed so as to project a detection light beam 73 in a perpendicular direction with respect to the surface of an object 72 and, with the light-receiving surface of a PSD 76 being disposed perpendicularly with respect to the axis of a detection light beam 73, a condenser 79 is disposed in an inclined position so that a reflected light 78 from the object 72 is invariably incident on the light-receiving surface 77 of a PSD 76. However, even in this displacement sensor 71, where the axis of the detection light 73 is perpendicular to the light-receiving surface 77 of PSD 76, no linearly displacement-dependent signal output can be obtained from the PSD 76 as shown in FIG. 12 because, assuming that the object 72 is displaced by a.sub.1 and a.sub.2, the amounts of shift of focus, b.sub.1 and b.sub.2, on the light-receiving surface 77 of PSD 76 are not proportional, viz. EQU b.sub.1 /a.sub.1 .noteq.b.sub.2 /a.sub.2.
The technological background of positioning devices is now described briefly. FIG. 13 shows a schema for positioning a magnetic head 82 with respect to a disk 81 in an optical/magnetic disk system. This is a positioning system known as the focus error system. Here, a laser light 83 from a write laser diode is condensed by optics 84 onto the disk 81 to detect the distance from the disk 81 from the amount of defocus of the laser light 83 and this detection signal is fed back to the magnetic head 82 to maintain the magnetic head 82 at a predetermined distance from the disk 81.
In such a focus error system, however, there are limits to the range of in-focus so that the disk and laser diode must be exactly positioned beforehand. For this reason, high assembling accuracy is required for the laser diode and other devices and the assembly of a magnetic disk involves a time-consuming operation.
The system for positioning a magnetic head as shown in FIG. 14 detects the absolute position of a magnetic head 82 with a displacement sensor 93, which comprises a light-emitting element 91 and a PSD 92 such as those as illustrated in FIG. 10, and calcualtes the distance d between the magnetic head 82 and the disk 81 to thereby indirectly detect the position of the magnetic head 82.
Since, in this system, the distance d between the magnetic head 82 and the disk 81 is determined indirectly by calculation, the system has the drawback of fairly large positioning error.
In the impact dot printer, the printing head must be positioned with respect to the printing paper. In the conventional practice, the printing head is once advanced into contact with the paper for positioning and, then, carried back away from the paper over a predetermined distance to set the head in a necessary position relative to the paper.
However, this system involves a large-stroke movement of advancing the printing head into contact with the paper and carrying it back and requires a device for sensing the timing of the printing head contacting the paper and a mechanism for idling the printing head so that it will cease to advance as soon as it contacts the paper. Therefore, the system architecture is inevitably complicated.